The field of the invention generally relates to repairing voids in asphalt pavement or surfaces, and more particularly relates to tack compounds and a method of repairing such asphalt pavement using microwave energy.
As is well known, asphalt materials are widely used for paving roadways, parking lots, pathways, and the like. In typical road construction, the roadway is prepared by laying a granular subbase of crushed stone or the like on compacted fill. Multiple layers or courses of asphalt material are then placed on the road bed. As each layer is applied, it is suitably leveled and compacted such as by a roller.
Asphalt material is typically a mixture of bitumen and stone commonly called aggregate that are heated and mixed together at a remote mixing plant. The hot mixture is then transported to the roadway and applied using paving machinery. The composition of each of the asphalt layers is dependent on the projected use of the roadway, but generally asphalt layers of coarse aggregate are applied first with a final or top layer of fine aggregate that is 2"-3" thick.
As is well known, asphalt pavement may be subject to localized erosion caused primarily by weather conditions and heavy traffic. Voids in asphalt pavement typically take the form of cracks or holes commonly referred to as potholes, and the depth of voids may be within the top layer or anywhere down through the coarse layers into the subbase. In the repair of roadways, it has been found beneficial to apply heat to the area being repaired during or after the repair operation. For example, a gas flame heater or a radiant heater may be used to heat the asphalt material and cause the material to soften. It has been found desirable to first spray a coating of hot tack into the pothole or crack before filling it with hot asphalt patch material. The tack serves as an adhesive interface and forms a bond between the patch material and the inner surface of the void. Unfortunately, most potholes evolve under weather conditions that are adverse to good bonding. For example, a roadbed surface in a northern state would likely be near freezing or below during winter; without adding considerable heat to the pothole, the spray of tack cools rapidly resulting in a poor quality bond.
Microwave energy has been used to heat asphalt material in situ (i.e. after being laid on the roadbed). For example, U.S. Pat. No. 4,594,022 to Jeppson describes the use of a sheet or layer of microwave reflective material such as a metal foil being applied below a top layer of asphalt material. The embedded sheet of metallic material acts as a microwave reflector to enhance the heating of the top layer of asphalt material. This is accomplished by reflection of the microwave energy from the metal foil layer. Jeppson teaches that microwave energy will typically penetrate into an asphalt material approximately 7-8". Although Jeppson's reflector is used to concentrate the microwave energy in the top layer, asphalt materials are still generally not very lossy; that is, asphalt materials are generally not readily susceptible to being heated by microwave energy.
In U.S. Pat. No. 4,849,020 to Osborne et al., lossy material is described being mixed with the asphalt material to provide a patch material that readily absorbs microwave energy and heats efficiently. Lossy microwave materials are those materials that absorb microwave energy by coupling with the electrical component, the magnetic component, or both components of the impinging microwave energy. The described lossy materials are semi-conductors, ferromagnetic materials, metal oxides, dielectric materials, metals in powder or particle form, and mixtures thereof. One drawback of this method is that when a large volume of asphalt is used, a proportionally large volume of lossy material such as ferrite is also required, and the cost may be high. Another drawback is that when a lossy material is mixed with the asphalt patch material, the depth of penetration is significantly reduced because the microwave energy is greatly attenuated by absorption near the surface of the pavement. As a result, the surface will get hot, but the region below the surface may stay cool. Accordingly, if the interface between the patch material and the roadbed is relatively deep, the tack layer may not heat effectively and a poor bond may result. Thus, the patch material may separate from the inner pothole surface. As often is the case, the patch material is then dislodged from the pothole by traffic, and the pothole returns to its original unrepaired form.